An international team of physicists, including many from the UK, has discovered a new fundamental difference between matter and antimatter. The team, working on the detector known as BaBar at the Stanford Linear Accelerator Center (SLAC) in California, has observed the intriguing effect known as direct Charge Parity (CP) violation in the disintegrations of heavy, short-lived subatomic particles called B Mesons.
At the birth of the Universe in the Big Bang, equal quantities of matter and antimatter should have been produced. Antimatter and matter mutually annihilate when they come into contact. So the equal quantities of the two forms of matter should have wiped each other out, leaving nothing. However, we live in a matter Universe. The existence of direct CP violation helps to explain Nature's preference of matter over antimatter and is a foundation for understanding why we are all here.
"This result determines directly for the first time the magnitude of the fundamental matter/anti-matter difference (or asymmetry) in Nature," says Paul Harrison, from Queen Mary University of London, and Chair of the BaBar-UK Steering Committee.
BaBar is a sophisticated 1,200-tonne detector, built and operated by a team of more than 600 physicists and engineers from nine countries**, including the UK. The detector records subtle distinctions between decays of B mesons and those of their antimatter counterparts, called anti-B mesons. Both are more than five times as heavy as the more familiar proton and survive just over a
trillionth of a second.
"These results will help us to more fully understand the huge inbalance between matter and antimatter in the Universe," said Professor Ian Halliday of PPARC, the UK's strategic science investment agency and funding body for the UK groups involved on Babar. "Not only have UK scientists made a significant contribution to Babar, but UK industry has supplied components worth over £8 million too, a clear indication of science delivering economic benefits to UK plc", added Halliday.
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This subtle difference between matter and antimatter was first seen more than 35 years ago in experiments with another particle, called the kaon. Now for the first time BaBar has shown that another type of particle exhibits this puzzling phenomenon, which should help physicists to understand the domination of matter over antimatter in the Universe. The new result is important, because unlike in experiments with kaons, the BaBar physicists have been able to measure directly a parameter in the Standard Model of particle physics.
The physicists used the detector to observe an unmistakable difference, or asymmetry, between the rates at which B and anti-B mesons decay into a special set of other, lighter particles. From these measurements, they calculate a parameter called sin 2beta ("sine two-beta"), which expresses the
degree of asymmetry between matter and antimatter. A non-zero value of this parameter is clear evidence for CP violation among B mesons.
The collaboration has reported its result in a paper submitted July 5 for publication in the leading scientific journal Physical Review Letters. In the paper, the team reports measuring a value of sin 2beta = 0.59 +/- 0.14, which is substantially different from zero. There are now less than
3 chances in 100,000 that the actual, physical asymmetry could be consistent with zero.
This result from BaBar is easily the most precise measurement of sin 2beta reported to date. Earlier measurements made at the Fermi National Accelerator Laboratory, at the Japanese National
Laboratory for High-Energy Accelerator Research (KEK) whilst consistent with the present result are not as accurate.
"We are now poised for further discoveries that should open up new directions for particle physics", says Stewart Smith, from Princeton University and Spokesman for the BaBar collaboration.
The precision of the BaBar result was made possible by the outstanding performance of the PEP-II "B Factory" at SLAC. Built in collaboration with the Lawrence Berkeley and Lawrence Livermore National Laboratories, this pair of 2.2 kilometer storage rings collides unequal-energy beams of electrons and their antimatter counterparts, called positrons. Since it began operating in
June 1999, the B Factory has produced more than 32 million pairs of B mesons, from which data the present BaBar result was extracted.
"The B Factory has performed beyond expectations, permitting the BaBar collaboration to make world-class measurements on B mesons," says SLAC Director Jonathan Dorfan, who played a
pivotal role in designing and building the particle collider.
The mysterious phenomenon of CP violation was first discovered in a 1964 experiment at the Brookhaven National Laboratory led by James Cronin and Val Fitch. Their group observed this behavior in decays of subatomic particles called K mesons, which are about one tenth as heavy as B mesons and live much longer; the two physicists shared a Nobel prize for the discovery.
Since then, several observations of CP violation have occurred in experiments with K mesons, but until the recent BaBar discovery, no other subatomic particles had clearly exhibited this exceedingly rare phenomenon. Having a second striking example of CP violation should aid theorists who are trying to understand what causes it.
Scientists are extremely interested in this puzzling behavior because it may help explain why there is such a preponderance of matter in the Universe. In 1967, Russian theorist Andrei Sakharov used CP
violation to suggest how the present matter-dominated Universe could have emerged from one that contained exactly equal amounts of matter and antimatter during the earliest moments of the Big Bang.
Notes for editors:
- sin 2beta should be written with "beta" as lower case Greek letter beta.
- BaBar should be written with the two letter Bs as capitals, and the other letters as small capitals.
** The BaBar collaboration includes physicists from 73 institutions in Canada, China, France, Germany, Great Britain, Italy, Norway, Russia and the United States. There are 65 UK members of the collaboration, from the Universities of Birmingham, Bristol, Brunel, Edinburgh, Liverpool, and Manchester; Imperial College London, Queen Mary University of London, and Royal Holloway
University of London; and the CCLRC's Daresbury and Rutherford Appleton Laboratories.
*** The Stanford Linear Accelerator Center is a national laboratory for high-energy physics and synchrotron-radiation research operated by Stanford University on behalf of the U.S. Department of Energy. The Department's Office of Science funded the construction of the B Factory at $177 million and contributed about 60 percent of the cost of the BaBar detector, with the remainder coming from foreign sources."Foreign contributions to this experiment, both monetary and scientific, have been absolutely crucial to its success," notes former BaBar spokesman David Hitlin of the California Institute of Technology.
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